Water intended for human use and consumption should be free of harmful chemicals and disease-causing bacteria or other microorganisms. A chlorine gas or a chlorine solution is often added to water for disinfection and control of microorganisms. However, testing for residual chlorine concentration after water treatment should be done because chlorine is known to react with organic matter in the water to form trihalomethanes (THMs), a suspected carcinogen.
Free chlorine is defined as the concentration of residual chlorine in water present as dissolved gas (Cl2), hypochlorous acid (HOCl), and/or hypochlorite ion (OCl—). Combined chlorine is defined as the residual chlorine existing in water in chemical combination with ammonia or organic amines which can be found in natural or polluted waters. Total chlorine is the sum of free and combined chlorine.
Commercially available multi-parameter instruments for water analysis include, for example, the LaMotte TC-3000 Tri-Meter, which measures turbidity, color and chlorine, or Hanna Instruments C 114 Turbidity and Chlorine Meter, which measures turbidity and chlorine. For chlorine evaluation both of these instruments use colorimetric methods when specific chemicals that change their color in the presence of chlorine are added to a water sample. Chlorine concentration can then be evaluated by intensity of color produced in the sample. Colorimetric kits and methods are developed by several companies such as CHEMetrics, LaMotte, Hach, Hanna for many analytes. Colorimetric test kits including as a pre packed set of chemicals are available, for example, under the trade name VISOCOLOR® Test kit for water analysis (http://www.cscjp.co.jp/VISOCOLOR_e.pdf) which allows for evaluation of multiple parameters and components in water. Colorimetric kits are developed for visual evaluation and for use with different optical instruments, such as spectrophotometers and photometers. The photometer PF-11 (VISOCOLOR®) which is recommended for use with a VISOCOLOR®Test kit is a single beam filter photometer having a filter wheel with 6 colored glass filters and manual wavelength adjustment. It uses six wavelengths—380 nm, 405 nm, 470 nm, 520 nm, 605 nm and 720 nm from a tungsten lamp. The wavelengths are chosen to match absorbance peaks for specific colorimetric reactions.
Colorimetric test kits having self-filling reagent ampoules are available from CHEMetrics http://www.chemetrics.com/selffill.html) when company introduced the self-filling reagent ampoules. The CHEMetrics Model V-2000 photometer uses one of three light emitting diodes (LED) to measure the optical density of a test ampoule after the water sample is mixed with reagent producing a specific color reaction.